1. Field of the Invention
The subject invention pertains to thermosetting bismaleimide resin systems. More particularly, the subject invention pertains to bismaleimide resin systems containing both a particulate thermoplastic toughener and a minor quantity of a low viscosity epoxy resin. Such resin systems exhibit surprisingly high compression after impact (CAI) strengths when used as the matrix resin in carbon fiber reinforced composites.
2. Description of the Related Art
Although many thermoplastics are tough, ductile materials, their use in structural materials has been minimal for several reasons. First, many of the thermoplastics do not have the required solvent resistance, thermal stability, and high softening points required in demanding aerospace applications. Second, the high temperature engineering thermoplastics are difficult to process, often requiring both high temperature and pressure to produce acceptable fiber reinforced parts.
For these reasons, and despite the proliferation and improvement of high temperature, high performance thermoplastics, thermosetting systems currently remain the important commercial resin systems. Of the thermosets available, by far the most common are the epoxies, the bismaleimides, and the cyanates. Each of these resin systems has its own unique set of physical and chemical attributes, but all are glassy, generally crosslinked systems which tend to be brittle. Thus attempts at toughening such systems have become increasingly important.
By the term toughness is meant resistance to impact induced damage. Toughness in cured neat resin samples may be assessed by the critical stress intensity factor, K.sub.1C, among others. Toughness in fiber reinforced composites prepared by laying up and subsequently curing numerous plies of prepregs is best assessed by measuring the compression strength after an impact of suitable energy. Generally, an impact of 1000 or 1500 in-lb/in (respectively, 4.45 and 6.68 kJ/m) is used, and compression after impact (CAI) values measured in accordance with Boeing test BSS 7260 on a quasiisotropic [+45/0/-45/90].sub.4s layup. Similar tests may be specified by other aerospace manufacturers.
Elastomers have been used with good success in toughening a number of thermosetting resins, particularly epoxy resins. Examples of such systems are given in Bauer, Epoxy Resin Chemistry II, Chapters 1-5, ACS Symposium Series 221, American Chemical Society, Washington, D.C., 1983. Both soluble and infusible elastomers have been utilized, the former generally increasing flexibility at the expense of physical properties such as tensile modulus, while the latter generally increase toughness without substantially affecting bulk properties. Both types of modification generally lead to lower thermal properties, an effect which can be minimized when polysiloxane elastomers are utilized.
Soluble thermoplastics have also been used, for example in the article by Bucknall and Partridge, "Phase Separation in Epoxy Resins Containing Polyethersulfone," Polymer 24 639-646 (1983). In Bucknall's examples, dissolution of up to 17 percent by weight of a polyethersulfone having a molecular weight slightly greater than 20,000 Daltons in an epoxy formulation increased toughness as measured by K.sub.1C by up to 50 percent. At the highest levels, phase separation was noted upon cure of the system, the resulting cured neat resin consisting of the glassy polyethersulfone discontinuous phase dispersed within a glassy epoxy continuous phase. With epoxy resins having an average functionality of four, no phase separation was observed, although the cured system still displayed enhanced toughness.
Dissolution of up to 80 weight percent of soluble polyimide PI2080 into the bismaleimide of bis[4-aminophenyl]methane was disclosed by Yamamoto in "Preparation and Characterization of Thermo-Plastic/Thermo-Setting Polyimide Blends," published in SAMPE Journal, July/August, 1985. However, resin systems containing high levels of dissolved polyimide are difficult to process and generally have little if any tack, an important consideration in the laying up of prepregs into composites. Furthermore, high levels of dissolved thermoplastic make fiber impregnation by the film method difficult.
Toughened systems have also been proposed which rely for toughness, on the use of oligomeric curing agents or monomers. Such monomers and curing agents have less crosslink density and thus are inherently more flexible, tougher systems. In U.S. Pat. No. 4,608,404, for example, epoxy resin systems containing an epoxy resin and an oligomeric amine terminated polyethersulfone is disclosed. Such systems were capable of providing composites having CAI (compression after impact, see infra) values of greater than 30 Ksi, particularly when diaminodiphenylsulfone (DDS) was used as a co-curative.
In U.S. Pat. Nos. 4,656,207 and 4,656,208, the principles of Bucknall and Partridge and of the '404 patentees were logically combined to provide epoxy systems employing DDS and greater than 25 percent by weight of a reactive polyethersulfone oligomer having a molecular weight of from 2000 to 10,000 Daltons. These epoxy systems cure into two phase systems having a glassy discontinuous phase dispersed within a glassy continuous phase as disclosed by Bucknall but utilizing a lower molecular weight, and thus more soluble and less viscous, polyethersulfone oligomer. Carbon fiber reinforced composites employing the resin systems of the '207 and '208 patents are able to achieve CAI values in excess of 40 Ksi. Other researchers have utilized analogous technologies with bismaleimide resins.
In U.S. Pat. No. 4,604,319, discrete films of thermoplastic, optionally containing up to 40 percent by weight thermosetting resin, are applied under heat and pressure to epoxy or bismaleimide prepregs containing carbon fibers as reinforcement. When such film faced prepregs are laminated together to form a composite, CAI values greater than 40 Ksi can be obtained. Unfortunately, such prepregs have not been accepted by the industry due to the possibility of a mistake during layup wherein two thermoplastic films might abut each other, promoting catastrophic interlaminar separation. Furthermore, such prepregs have little tack, and thus make composite layup difficult.
In European patent EP-A-0 252 725, elastomeric interlayers are formed in situ by the filtering out of discrete, infusible particles by the fiber reinforcement because the particles are larger (10-75 .mu.m) than the fiber interstices. Prepregs such as these and composites formed therefrom have the capability of having CAI values in the 40-50 Ksi range, but may suffer from lower properties at elevated temperatures.
In European patent EP-A-0 274 899, the addition of thermoplastics, preferably in the form of solid, spherical particles, to thermosettable resin systems is said to cause an increase in toughness in carbon fiber laminates. Examples of thermoplastics are polyamideimides, polybutyleneterephthalate, and nylon, with transparent nylons being preferred. When particles greater than 2.mu.m in diameter are utilized, the thermoplastic is concentrated in situ onto the outside of the prepreg as in EP-A-0 252 725. When particles having a size less than 2 .mu.m are used, the thermoplastic remains homogenously dispersed within the prepreg.
U.S. Pat. No. 4,131,632 indicates that bismaleimides and epoxy resins may be combined, but that the content of bismaleimide must be limited to no more than 30 weight percent due to the incompatibility of the bismaleimide and epoxy resins. U.S. Pat. No. 4,212,959 also discloses these drawbacks of combination epoxy and bismaleimide resin systems, as well as the further drawback that such systems exhibit high shrinkage during cure. Such resin systems would be expected to result in distorted composites and/or severe microcracking when utilized as thermosetting matrix resins in structural composites.
In U.S. Pat. No. 4,743,647, epoxy resins are one of many suggested comonomers which may be added to bismaleimide resins, particularly with the use of diamine epoxy resin curing agents. However, no guidance as to the selection of epoxy resins or curing agents and no examples of such use are given. In U.S. Pat. No. 4,510,272, the use of bismaleimides in epoxy resin systems is taught, but once again, no direction as to selection of epoxy resin is given. The epoxies are cited as improving the high temperature water resistance. No mention of toughness as reflected by resistance to impact is mentioned. Furthermore, the systems exemplified all contain about 30 weight percent or more of the epoxy resin, generally in conjunction with an amine curing agent.